Astrophysical preference between constant and variable stellar anisotropy joint models for the Cosmic Horseshoe lens

Ascertain which joint lensing-and-dynamics mass model should be astrophysically preferred for the Cosmic Horseshoe lens galaxy: (i) the fiducial configuration with constant stellar anisotropy and an elliptical Navarro–Frenk–White dark-matter halo (scale radius fixed at 10 times the galaxy’s effective radius) plus a central supermassive black hole, or (ii) the alternative configuration with a radially varying stellar anisotropy profile (three free anisotropy parameters) while keeping the other mass components identical.

Background

The paper jointly models strong gravitational lensing and stellar dynamics of the Cosmic Horseshoe lens galaxy, inferring an ultramassive black hole. To assess robustness, the authors compare multiple mass models, including a fiducial model with constant stellar anisotropy and an alternative model with a variable anisotropy profile.

While the variable-anisotropy model (M3) yields slightly higher Bayesian evidence than the fiducial model (M1), the difference is not decisive. The authors explicitly state they lack sufficient prior knowledge to determine which model should be astrophysically preferred, highlighting an unresolved question about the correct anisotropy treatment in this system.